Electroluminescence generates visible light in a manner distinct from light emission based on other physical principles such as blackbody radiation. It has the unique ability of generating optical radiation over a broad spectrum and from a source of either large or area without resorting to high temperature.
Electroluminescence was discovered shortly before the Second World War by Destriau in a system which consisted of a phosphor (a mixture of zinc sulfide and zinc oxide) suspended in a dielectric medium (castor oil) between two electrodes. For a number of reasons, it has been actively investigated by many scientists and engineers since 1946. As a result of such efforts, the fundamental principles of electroluminescence are quite well understood, i.e., it is the result of collision excitation of certain phosphors by an alternating electric field. Accordingly, subsequent effort have mainly been expended on the preparation of making phosphors with selected characteristics or on improvements in the manufacturing methods.
Electroluminescent light sources are sometimes identified in the literature as electroluminescent lamps. In the discussions that follows, the original terminology in the literature will be used even though an electroluminescent light source is more analogous to a light bulb than a lamp.
Commercially available electroluminescent light sources are often inadequately insulated, i.e., their electrical conducting components may not be properly protected against moisture or electrical leakage. In view of the unique characteristics of electroluminescent light, however, it is not difficult to visualize the advantage of portable self-contained electroluminescent lighting systems, e.g., their use as a night light, a low intensity illuminator, an ornament for a garment, or an enjoyable toy for children of all ages.
In at least one special application involving the possibility of saving a human life, a portable electroluminescent lighting system has been described in Brain U.S. Pat. No. 5,034,847. In Brain, a series of electroluminescent lights are connected to each other with electrical leads and sealed in a protective envelope which may be made from silicone, fluorosilicone, or a fluorocarbon elastomer. This sealed unit is then connected through a cable to a water-tight power supply constructed with a series of connectors, O-rings, etc. By using such a design, additional components such as a water-sensing switch and a radio-beacon circuit may also be incorporated, The light source operates automatically when exposed to moisture, and aids the efforts of a search party in a rescue operation.
The electroluminescent lighting system in Brain appears to be a valuable addition to navigation safety, but would be expensive in view of the manufacturing steps required for the water tight power supply unit. Moreover, the existence of three separate components (power supply, cable, light source) would make it inconvenient or even impractical to be used as a portable light, a novelty decoration, or in other consumer applications.
Methods for insulating an electroluminescent light source without insulating the power supply and the electrical terminals also have been disclosed in the literature. In Calamia U.S. Pat. No. 4,999,936 is described a light source which comprises a laminated structure of a flexible non-conductive material, a conducting film, a phosphor coating which may be tinted to produce a variety of light, a second transparent conductive film, and an open ended conductive trace. The laminated structure, in turn, is protected by an laminating film and inserted into a clear protective cover. Electrical connection is accomplished by cutting a slit into the cover and said encapsulating film and attaching a wiring clip onto the open ended conducting trace. The electrical connection area and the conducting trace exposed through the slit therefore remain un-insulated.
A somewhat similar approach is described in Devol and Kirk U.S. Pat. No. 3,177,391. In Devol and Kirk, a conventional flexible electroluminescent cell or lamp is provided with a stiffening means comprising a layer of open-weave cloth material partially embedded in the outer side of the thermoplastic cover layer of the cell or lamp, and an outer stiffening overlay comprising preferably of a different thermoplastic sheet material having a softening temperature appreciably below that of the first thermoplastic layer. In this matter, the plastic stiffening sheet is laminated to the cell at a temperature appreciably below the softening temperature of the outer-layer of the cell and thereby avoiding any damage of the cell by the heat attending such laminating operation. Ribbon-type conductors which project laterally from edge of the outer envelope of stiffening overlay are, however, needed so that the cell can be connected to a power supply. In other words, the electrical connections remain un-insulated as in the light source in Calamia.
An alternative approach for insulating an electroluminescent light source is described In Schrank U.S. Pat. No. 4,618,802. It details a method to hermetically seal an electroluminescent or other thin film device in which the device includes a thin film matrix supported on a glass substrate. The glass substrate is mounted in a frame, with a frit seal provided between the glass substrate and the frame, and a cover secured to the frame by a weld or solder joint. To maintain structural integrity of the system, materials with substantially similar coefficients of thermal expansion are used to form surfaces bounding each seal so as to maintain structural integrity of the system. This method would be applicable, however, only to rigid electroluminescent light sources. It can not be used for the more common flexible, plastic based, electroluminescent light sources such as the one described in Fridrich et al U.S. Pat. No. 2,945,976 and which have been commercially available from many sources at least since the early 1980's.
For many potential consumer applications, however, insulating only the electroluminescent light source can only be considered a partial solution. An electroluminescent lamp requires electric power in the form of an alternating current, be it directly from a generator or from a battery through a D.C. to A.C. inverter. Typically, the A.C. voltage is on the order of 100 volts or higher. Accordingly, if any un-insulated conducting area is exposed to moisture or touched, lost of power through leakage or unpleasant shock may occur. For stationary subjects such as exit signs in buildings, additional protection may be afforded by positioning the light source away from moisture or possibility of contact. Such an approach is, however, of little value if the electroluminescent lamps are to be used in applications such as a low intensity portable light, an ornament, or a toy for children.